Non-polarized memory-switching characteristics of ZnTe thin films

ZnTe thin films were prepared by the conventional vacuum-deposition method. Polycrystalline thin films could be deposited on a glass substrate at substrate temperature between 20 and 450°C. Epitaxial thin films could be obtained on GaAs and InAs single crystals at substrate temperatures higher than 200°C.Memory switching could be observed in metal-ZnTe-metal sandwich devices prepared on glass substrates. Both switching transitions, from off to on and on to off, could be performed without changing the bias polarity if a suitable value of series resistance was selected for each transition. Typical values of the series resistor were 20 KΩ for off to on and 200 Ω for on to off transitions. Transition from off to on is explained as a result of the formation of a filamentary path in the off state, followed by the formation of a metallic filament in the on state. Transition from on to off is considered a result of thermal rupture of the metallic filament.     

Cu掺杂ZnO薄膜光学性质的研究

为了制备结晶质量好的Cu掺杂ZnO薄膜,研究其结构和光学性质,采用脉冲激光沉积方法,在Si衬底上选择不同的衬底温度来制备薄膜。实验成功制得了结晶质量较好的Cu掺杂ZnO薄膜。利用X射线衍射仪、扫描电子显微镜和荧光分光光度计对样品进行了测量和分析。所制备的样品均表现出高度的c轴择优取向,衬底温度为300℃时,薄膜表面形貌均匀致密;在样品的光致发光谱中,发现样品除了在380nm附近出现紫外发光峰外,在460nm附近出现了蓝光发光峰,真正意义上实现了ZnO薄膜的蓝光发射。结果表明,衬底温度对其晶体质量有较大影响。     

Conduction mechanisms in the off-state of thin ZnTe films

Memory switching in ZnTe layers was observed. The electrical conduction in the non-ohmic off-state is analysed. D.c. and a.c. measurements demonstrated the existence of thin (50 Å) contact barriers at the Al-ZnTe interface. For voltages lower than 1V conduction is governed by Schottky emission over that barrier. For higher fields the most probable mechanism is a modified Poole-Frenkel mechanism which is equivalent with field-assisted hopping of holes from traps with a density of 5×10¹⁸ to 10²⁰cm^?3. No evidence was found for space-charge injection.     

Optical properties of PbS thin films

The complex dielectric function of PbS thin films is studied by spectroscopic ellipsometry in the spectral range from 0.74 to 6.45 eV at a temperature of 293 K. The critical energies are determined to be E ₁ = 3.53 eV and E ₂ = 4.57 eV. For both energy regions, the best fit is attained at the critical point 2D (m = 0). In addition, the Raman spectra and the optical-absorption spectra of PbS thin films are studied. From the dependence of the quantity (αhν)² on the photon energy hν, the band gap is established at E _g = 0.37 eV.     

半导体氮化物AlInN的光学性质

测量了AlInN薄膜(包括InN和AIN)的透射和反射光谱,结合四层透射和反射模型得到AlInN的一系列变温光学性质,包括吸收系数、能带带隙、乌尔巴赫带尾参数、折射率等等.采用一套经验公式,描述InN薄膜在本征吸收区和乌尔巴赫吸收区的吸收系数.发现带隙以下,AIN薄膜的折射率遵守Sellmeier经验公式.用基于态密度...     

Optical transmission in thin films of vanadium compounds

Thin films of five vanadium compounds/composite： （1） VO2（3-fl） （3-fl = 3-Hydroxyflavone）, （2） VO（pbd）： （pbd = 1-Phenyl-l, 3-butadione）, （3） VO（dbm）2 （dbm = Dibenzoylmethane）, （4） VPc （Vanadyl Phthalocyanine） and （5） V2O5-PEPC （V2O5- poly-N-epoxypropylcarbazole composite）, were deposited by the dropcasting method from the solution in benzene. The transmittance-irradiance relationships were investigated and the transmission in the visible spectrum and optical images were obtained as well. It was found that the transmittance of the VO2（3-fl）, VO（pbd）2, VO（dbm）2 and VPc, was practically independent of the irradiance; whereas the transmittance of V2O5-PEPC decreased by 4% for thin and 9% for thick films with an increase of the irradiance.     

Optical response of gamma irradiated-arsenic selenide thin films

Gamma(γ)-radiation induced effects in thin films of arsenic selenide (As₂Se₃) were investigated for possible applications in γ-radiation environment and/or γ-radiation dosimetry purposes. Thin film samples were fabricated using thermal evaporation. Optical properties of As₂Se₃ thin films were studied before and after irradiation. As a result of γ-irradiation, photodarkening of samples was observed for doses up to 10⁴ Gy (10 kGy). Absorption coefficient was found to increase with increase of irradiation dose. Optical energy gap was found to decrease while the localized band tail width was found to increase with dose. SEM was used to examine irradiated films; small nucleation of the crystalline phase could be distinguished. Nucleation increases with dose indicating some local structural changes due to irradiation. Values of absorbance or absorption coefficient in the absorption edge region are suitable control parameters of the proposed dosimeter material. The observed changes in the optical properties suggest that As₂Se₃ thin film may be considered as an effective material for room temperature real time γ-radiation dosimetry for the relatively high dose industrial applications such as sterilization.     

Optical absorption behaviour of vanadium pentoxide thin films

Thin films of vanadium pentoxide have been prepared by the electron beam evaporation technique over a wide range of substrate temperatures. Spectrophotometric investigations have been made on these films in the wavelength range 350–1500 nm to determine the nature of the optically induced transition and the optical band gap. A better fit for the direct forbidden transition than for the indirect allowed transition is observed in all the films. The optical band gap is found to decrease with increasing substrate temperature and the subsequent appearance of a broadband absorption at about 1000 nm is attributed to the oxygen vacancies in the films. The effect of heat treatment in vacuum and oxygen ambient on the optical properties of these films is also reported. © 1997 John Wiley & Sons, Ltd.     

Effects of Cu incorporation on physical properties of ZnTe thin films deposited by thermal evaporation

The present paper reports on a systematic study of the Cu doping effect on the optical, electrical and structural properties of ZnTe:Cu (Cu=0, 6, 8, and 10 at%) thin films. Polycrystalline Cu-doped ZnTe thin films were deposited on glass substrates at room temperature by thermal evaporation. A detailed characterization of the Cu-doped ZnTe films were performed by X-ray diffraction (XRD), Spectrophotometry, Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy. XRD of the as-deposited Cu-doped ZnTe films belong to single-phase cubic structure of ZnTe with preferential orientation along (111) planes revealed minor effect of Cu content. The interference pattern in optical transmission spectra was analyzed to determine energy band gap, refractive index, extinction coefficient and thickness of the films. Wemple–DiDomenico and Tauc's relation were used for the determination and comparison of optical band gap values. The formation of ZnTe and Cu-doped ZnTe phase was confirmed by FT-IR. AC conductivity in a frequency range of 0–7 MHz has been studied for investigation of the carriers hoping dynamics in the films. Raman spectra indicated merely typical longitudinal optical (LO) phonon mode of the cubic structure ZnTe thin film at 194 cm⁻¹ because the excitation energy is well above of the optical band-gap of the material and exhibited a blue-shift from 194 to 203 cm⁻¹ with Cu which could be associated to the substitution of Zn atom with Cu at the lattice sites.     

Optical waveguiding and modulation in zinc-sulphide thin films

Some guiding and electro-optic properties of highly ordered (111)-oriented single-crystal zinc-sulphide thin films grown on silicon are reported. A simple relative-phase modulator is described. At a wavelength of 633 nm, the halfwave voltage is 58 V and the attenuation less than 6 dB.     

Optical coupling between thin films and circular fibres

Techniques are proposed for launching light into circular glass-fibre waveguides from prism couplers and thin-film optical waveguides. To make the evanescent field of core modes available, the fibre cladding is etched away, or its refractive index modified by ion exchange.     

Optical and electrochromic properties of annealed lithium-molybdenum-bronze thin films

Optical band gaps, Urbach inverse slopes, and coloration bands of various samples of annealed, microcrystalline Li_xMoO₃-bronze thin films in the concentration range 0<x<0.6 were determined over the photon energy range from 0.4 eV to 4.2 eV. On investigation, it is learned that the measured, optical band gaps do not shift rigidly over the annealing temperature range 293≤T≤423 K and, therefore, do not reveal the Burstein-Moss effect or reflect any stable, crystallographic phase transformation during any investigated annealing cycle. A model relating the temperature-dependent optical gap to the real part of the refractive index has also been developed, and this model fits very well to the annealed data within a maximum error of about 20%. Next, using an oscillator model, a phonon energy of ∼0.08 eV was obtained, which is very close to the characteristic phonon energy of the material, MoO₃. Using this model, it becomes more certain that the contributions to the Urbach absorption edge for the annealed-molybdenum bronzes are coming from the structural and compositional disorder. In another finding, it was found that the absorption-peak energy for the annealed data was about 1.5–1.6 eV, which is still broad and asymmetrical, and therefore, it is almost of the Mo⁶⁺ (or Mo⁴⁺)-Mo⁵⁺ intervalence or polaronic type. Using the polaron model, the half-bandwidth of the coloration bands of investigated, annealed Li_xMoO₃-thin films was found to be almost constant, which is consistent with the nonrigid band behavior.     

Optical properties of chemical bath deposited CdS thin films

CdS thin films have been prepared by chemical bath deposition. As-deposited films are cubic and show a sulfur deficiency. From the transmittance and reflectance data analysis direct band gaps (E_g) ranging from 2.180 to 2.448 eV have been obtained. Air and vacuum annealed samples show a decrease in the band gap. The refractive index (n) lies in the range 1.61–2.34. A dependence of band gap on composition has been observed and the possible reasons are discussed.     

Optical characteristics of twisted nematic liquid-crystal films

A twisted nematic liquid crystal is shown by theory and experiment to rotate incident plane-polarised light by ?/2 only at specified wavelengths. At other wavelengths, elliptically polarised light is produced. This action determines the contrast ratio of some twisted nematic devices.     

The structural,optical, and electrical properties of vacuum evaporated Cu-doped ZnTe polycrystalline thin films

We have studied the structural, optical, and electrical properties of thermally evaporated, Cu-doped, ZnTe thin films as a function of Cu concentration and post-deposition annealing temperature. X-ray diffraction measurements showed that the ZnTe films evaporated on room temperature substrates were characterized by an average grain size of 300Å with a (111) preferred orientation. Optical absorption measurements yielded a bandgap of 2.21 eV for undoped ZnTe. A bandgap shrinkage was observed for the Cu-doped films. The dark resistivity of the as-deposited ZnTe decreased by more than three orders of magnitude as the Cu concentration was increased from 4 to 8 at.% and decreased to less than 1 ohm-cm after annealing at 260°C. For films doped with 6–7 at.% Cu, an increase of resistivity was also observed during annealing at 150–200°C. The activation energy of the dark conductivity was measured as a function of Cu concentration and annealing temperature. Hall measurements yielded hole mobility values in the range between 0.1 and 1 cm²/V·s for both as-deposited and annealed films. Solar cells with a CdS/CdTe/ZnTe/metal structure were fabricated using Cudoped ZnTe as a back contact layer on electrodeposited CdTe. Fill factors approaching 0.75 and energy conversion efficiencies as high as 12.1% were obtained.     

Optical third-harmonic generation from Langmuir-Blodgett merocyanine dye thin films

Third-harmonic generation measurements have been made on LB Y-type multilayers of a protonated merocyanine dye. The third-order nonlinear susceptibility was deduced by observing the interference produced by changing the angle of incidence through the sample and referencing the signals to those from the substrate alone.     

Optical properties of CdSxTe1−x polycrystalline thin films

Thin films of CdS_xTe_1−x (0≤x≤ 1) have been prepared by vacuum evaporation from solid solutions. Rutherford backscattering spectrometry has been used to determine the thickness of the films, which is in the range 8–50 nm, and x-ray diffraction analysis has been used to determine the phase. The refractive index and extinction coefficient of the films has been calculated from reflectance and transmittance measurements for the wavelength region 250–3200 nm. Polynomial functions are given for each sample, which describe the variation in refractive index and extinction coefficient over the entire wavelength range. Least squares fitting to the absorption spectra revealed that the films all have a direct band gap, although photon energies required for indirect transitions have also been found. CdS_0.8Te_0.2 is found to have the lowest absorption coefficient at energies greater than 2.1 eV.     

Optical properties of YBaCuO thin films at Far-Infrared monochromatic light

By using monochromatic sources of radiation we measured the reflectivity and transmission vs temperature behavior of YBaCuO thin film in the Far Infrared region in the range 300-20 K. A smooth change of optical properties is observed from critical temperature down to 40 K with an evident reduction of the losses for all the frequencies we examined. To analyze the use of YBaCuO films as semireflecting elements in submillimeter devices we made a preliminary study of the optical properties of the substrate in the same conditions. The permanence of the sample in a superconducting phase can be deduced from its room-temperature transmittance, as it changes drastically if some degeneracy occurs in the film.     

Optical properties of ZnO thin films prepared by sol-gel process

The present study focused on ZnO thin films fabricated by sol-gel process and spin coated onto Si (1 0 0) and quartz substrates. ZnO thin films have a hexagonal würtzite structure with a grain diameter about 50 nm. Optical properties were determined by photoluminescence (PL) and absorption spectroscopy. The absorption spectrum is dominated by a sharp excitonic peak at room and low temperatures. At room temperature, two transitions were observed by PL. One near to the prohibited energy band in ultraviolet (UV) region and the other centered at 640 nm, characteristic of the electronic defects in the band-gap. The spectrum at 6 K is dominated by donor-bound exciton lines and donor-acceptor pair transition. LO-phonon replica and two-electron satellite transitions are also observed. These optical characteristics are a signature of high-quality thin films.     

Optical spectroscopy of thin C60:CdS composite films

The fabrication procedure, structure, and optical spectroscopy data for thin composite films based on a fullerene material with a 0.5–10% cadmium-sulfide impurity content are reported. The technology for producing structures with a composition gradient in a single vacuum cycle is developed. The samples are studied by atomic-force microscopy, Raman spectroscopy, and photoluminescence measurements. At the optimal CdS content, charge transfer to fullerene molecules is observed. This effect is apparently responsible for the zero-barrier polymerization of the films. In the photoluminescence spectra of the composite films, a 0.09–0.11 eV shift of the peak to shorter wavelengths and an increase in the photoluminescence intensity in comparison to the photoluminescence peak position and intensity in pure fullerene samples are detected. The composite structures with a CdS-content gradient through their thickness exhibit rectifying current-voltage characteristics.